1. Field of the Invention
The present invention relates to a method of operating a combined-cycle plant, essentially comprising a gas turbine, at least one generator and a steam circuit, this steam circuit essentially comprising a heat-recovery steam generator, at least one low-pressure and/or intermediate-pressure steam turbine and a high-pressure steam turbine.
2. Discussion of Background
EP-0 379 930 B1 has disclosed a combined-cycle plant which is characterized by a single-shaft arrangement. Here, the gas turbine and steam turbine are rigidly coupled to one another. The heat-recovery steam generator belonging to the steam circuit of this combined-cycle plant is operated as a dual-pressure boiler. The problem with such an arrangement is that the gas turbine must be prevented from going into overspeed in the event of load shedding and from then having to be shut down. On the contrary, it ought to go into isolated operation (=station-service operation) until it can be synchronized with the network again. However, this is difficult to carry out insofar as, on the one hand, the inflow of steam to the steam turbine has to be interrupted, otherwise the drive torque becomes too great and the plant consequently goes into overspeed, and, on the other hand, a minimum steam flow has to be maintained as cooling steam through the steam turbine in order to prevent the steam turbine firmly coupled to the gas turbine from heating up. Accordingly, provision is made here to maintain the cooling of the steam turbine with a quantity of steam prepared in an auxiliary unit, or to not completely close the valves of the steam turbine. Basically, however, the flow rate through the steam turbine is only reduced, so that the torque resulting therefrom, which leads to difficulties for the plant, has to be absorbed by other complicated control actions.